1. Field of the Invention
This invention relates to an apparatus and a method of making and using the apparatus for monitoring the integrity of a storage tank. In particular, the apparatus provides a conduit connected to the interstice between an inner and outer wall of a tank and connected external to the tank to so as to allow for detection of a breach of the integrity of either the inner or outer wall of the tank.
2. Description of Related Art
Commercial and industrial liquids of all types are stored in underground storage tanks. The tanks are generally cylindrical in shape and usually have a capacity in the range of 500 to 20,000 gallons or more. Such tanks are generally made of either metal (usually steel) or a fiber reinforced resinous material.
Because the liquids stored in such tanks are often hazardous, and thus can cause severe environmental damage and greatly impact the lives of people living, working, and recreating in nearby areas, careful attention to the potential for leaks from such tanks must be exercised. Due to these potential problems from leaks, safer storage tanks have been designed with a double wall, such that a breach in the integrity of either of the inner or outer wall alone will not allow a leak of the liquid contained in the tank. The use of such double-walled tanks (or equivalents thereof, wherein some sort of secondary containment is provided for an otherwise single-walled tank) is increasingly being mandated by government regulation.
In one alternative tank structure that provides added safety from the hazards of leaking storage tanks, a liner is installed in a tank that has been in use and is already in the ground. Certain of these liners can be installed without removing the tank from its underground position. Such a lining can be significantly more economical to install as compared with removal and replacement of the single-walled tank with a new double-walled tank.
In most of the safer structures wherein a tank has a double wall or a liner of some form, there exists a space between the inner wall and the outer wall or the liner and the outer wall, or in some cases between two walls of the liner. This space is often referred to as an interstice. The interstice is a volume—defined between two walls of the tank, two walls of the liner, or between walls of the tank and the liner—that may be wholly or partially evacuated, may be filled with a gas, such as air, or may be partially or essentially wholly filled with a liquid or solid material. An example of a solid filler material is a fibrous batt.
Because a breach in either of the walls that form the interstice is the first step towards a break in the tank that will allow a leak of the tank's hazardous contents, the monitoring for a breach in one of these walls is an important step to preventing leaks. If the breach of a first wall can be detected, a remedial action can be taken prior to a breach of the second wall that would allow the leak into the external environment. Monitoring for a breach in either an inner or outer wall about the interstice is readily accomplished by monitoring the characteristics of the interstice.
For tanks that are retrofit with an inner tank wall or liner, there have previously been developed at least two methods for allowing monitoring of the interstice. Both methods employ a conduit that is attached to the inside of the tank at the top of the tank directly under an entry bunghole. Additionally, both methods employ a conduit that is rigidly attached to the tank at both of its two ends.
In the first, a generally semicircular conduit is constructed and laid along an inner circumference of the tank from the top to the bottom of the tank, after the inner wall or liner has been constructed in the tank. The conduit covers over both an opening in the liner at the bottom of the tank and an entry bunghole at the top of the tank, providing fluid communication from the interstice through the conduit to the entry bunghole. After placing the conduit along the inside of the tank, a layer of fiberglass and resin is applied over the conduit to secure the conduit in place.
In the second method, the lining is first constructed inside the tank. Then, a straight, fiberglass reinforced polymer (FRP) tube is coupled to the lining at the bottom of the tank at a location where a hole in the lining passes through to the interstice. On an end opposite the bottom of the tank, the tube is connected to a fiberglass sleeve that allows for a further connection to a length of flex hose, which rises the remaining height to the top of the inside of the tank where a double tap bushing is used to rigidly attach the flex hose to the tank through two threaded connections—flex hose to double tap bushing and double tap bushing to tank—directly below an entry bunghole.
Each of these methods has problems that make their use inconvenient, at best. One of the most significant problems with the semi-circular conduit is that it is difficult to get a probe or other monitoring device through the curved conduit after construction is complete. The most efficient manner of installing a probe in the needed location at the bottom of the tank is the use of a specially designed device, called a probe taxi, which delivers the probe through the curved conduit. Use of the taxi, significantly increases the expense of the installation. The second method, using a straight FRP tube, is generally too rigid a construction, even with the use of the flex hose. The conduit (including the double tap bushing, tube, and hose) in this method often does not maintain its closed path, but will break under the stress provided in the normal course of emptying and refilling the tank with liquid. As the tank flexes and its height changes, the tank will apply strain along the longitudinal direction of the conduit that cannot be absorbed by the conduit since the two ends of the conduit are rigidly attached at the top and bottom of the inside of the tank. This longitudinal strain can break the conduit. A broken conduit does not allow for monitoring of the interstice, since as a result of the fluid communication between the conduit path and the interstice, a breach in the conduit is similar to a breach in a wall of the interstice.